In systems using electromagnetic radiation, it is often useful to have some influence on the intensity of the electromagnetic radiation that is output by the system. In e.g. an X-ray system used for medical purposes, the intensity of the X-rays emitted often needs to be adjusted to the specific medical applications the radiation is used for. Another example is the use of light in optical systems such as in projection systems used for projecting strongly differing images. A further example are projection systems wherein more than one projector is combined. For example in simulation systems, it is highly desirable for all projectors in the system to have a comparable light output. This, amongst others, increases the homogeneity of the overall projected images. Therefore, it is an advantage if the generated light intensity for every single projection apparatus can be controlled and adjusted easily.
In some cases, the radiation output can e.g. be controlled by changing the power of the radiating system. However, very often this is either not possible or not possible in an efficient way, i.e. for example not easily or economically controllable. The use of a dimming means to limit or prohibit a part of the radiation, such as e.g. the light from the illumination system to be projected on the screen, may then offer a solution.
Often it is highly desirable to use one and the same radiation system for different purposes, whereby at one moment in time the radiation output should be high and at other moments the radiation output should be drastically reduced. A typical example is the use of a projection system for simulation purposes, wherein widely different situations need to be simulated. This can e.g. comprise simulation of military airplanes flying above hostile territory during day or at night. To simulate night conditions, reducing the signal provided to the projector, e.g. to 5% of its original value, will on the one hand lead to the screen looking dark as desired, but will at the same time also reduce the bit depth of the signal. This leads to details being no longer visible, which is disadvantageous for the simulation. By adjusting and controlling the light output obtained from the illumination source using a dimming means, no bit reduction occurs and thus the quality of the simulations remains.
Reduction of the E.M. radiation intensity using dimming means is known from different systems.
US-2003/0086265 describes a projection system with a dimming means comprising mechanical means to substantially block the light. The projection system comprises two fly-eye lenses and the dimming means can be positioned either in front of the first fly-eye lens, in between the two fly-eye lenses or after the second fly-eye lens. The invention is based on mechanical blocking of part of the light.
US-2003/0072161 describes a projection system wherein color filters and dimming means are provided to regulate the brightness on a corresponding screen. Preferably, plural evenly-spaced dimmer blades are used to block the light. A typical dimmer blade can be for example claw shaped.
Other typical prior art dimming means 102, 104, 106 at present commonly used in systems using electromagnetic radiation, such as e.g. projection systems, are illustrated in FIG. 1. All these dimming means typically comprise at least one first area 108 that is completely blocking an electromagnetic radiation beam such as e.g. a light beam impinging on the dimming means 102, 104, 106 and at least one second area 110 that is completely or substantially completely transmitting the electromagnetic radiation beam such as e.g. a light beam impinging on the dimming means 102, 104, 106. The latter can be done by either providing a substrate transparent to the electromagnetic radiation beam used or by not introducing any material for dimming. From the prior art documents mentioned above and from the design of the typical prior art dimming means 102, 104, 106 it can be seen that dimming typically is performed by blocking exterior parts of the cross-section of the electromagnetic radiation beam. These mechanical dimming means have shortcomings: the transition from transmitting to blocking the electromagnetic radiation is abrupt, and can be represented by a discrete transmission function jumping from 100% to 0% at once, leading to non-homogeneity in radiation intensity on the screen. This is illustrated in the following example.
By way of example, a more thorough description will be given of the commonly used dimming means used for light in projection systems. Nevertheless the general principles relate to each of the different types of electromagnetic radiation of the electromagnetic spectrum. The use of a dimming means 200 set-up in a projection system is illustrated in FIG. 2a. The projection system may e.g. have a light source with an elliptical reflector 202, which is focussed substantially close to the entrance of an integrating rod 204, a further processing means 210 and a screen 212. The dimming means 200 comprises e.g. 2 opaque—i.e. non-transparent—plates 206, 208, e.g. metal plates, that can be shifted with respect to each other in a direction substantially perpendicular to the direction of a light beam, and that allow to block exterior parts of the cross-section of the light beam. The reason for positioning of the dimming means 200 in front of the integrating rod 204 is the integrating nature of the rod 204. All light which enters the rod 204 is mixed so as to be uniform at the rod exit.
Nevertheless, as can be seen in FIGS. 2a and 2b, the dimmer will cut out light rays which in general have high angles of incidence. This leads to a reduction of the number of reflections in the integrating rod and therefore the screen uniformity will drop. This will even be worse for a spherical dimming aperture, such as demonstrated on the bottom right example of FIG. 1. In addition, filtering out one specific part of the angular space might introduce other artifacts, as e.g. commonly used dichroic coatings have spectral characteristics that vary strongly with the angle of incidence of the incoming light ray. A color change can therefore result from dimming.
Another option is to locate the dimming means behind the integrating rod. If the dimming means 200 would be located closely behind the exit of the rod 204, or close to the light valve, the dimming would be immediately visible on the screen 212 as a shadow. It would lead to severe non-homogeneities in the brightness distribution on the screen 212. The only good position to put a dimming means 200 in the relay optics (i.e. the optical path between the exit of the integrating rod and the light valve such as e.g. an LCD) is in a so-called aperture plane. This is an imaginary surface where the intersection points of the light rays are directly related to the angles of the rays on the display, being part of the further processing means 210. This is in fact the opposite of a surface such as the exit of the rod 204 and the light valve itself, where a position in the cross section of the light beam really corresponds to a position on the screen 212, as actually the display is an image of the integrating rod, and the screen 212 displays an image of the display itself. In other words, when the dimming needs to be done in the relay optics of a projection apparatus, dimming should always be performed in or close to the aperture plane of the projection system, in order not to affect the brightness homogeneity on the screen 212. Nevertheless, as the aperture plane is a very favorable position to put all different kinds of means for manipulating the image to be displayed—due to the anti-correlation of the position at this place with the position on the screen 212, the means cannot be seen as it is smeared out—such as for example contrast enhancing means or polarization recuperation means, it is unlikely that the dimming means 200 can also be located in the aperture plane.
In an alternative dimming means, described in U.S.-2003/0035290, the dimming is performed by gradually dimming the light beam over its complete cross-section. The dimming means comprises a spatially-modulated, variable-density, reflectively-coated dimming wheel which allows gradually dimming between 0% and 100% of the maximum light output of a corresponding projection system. The dimming is based on a reflective coating having a dotted pattern, the reflectivity of the coating thus varying along an appropriate path on the wheel. The density variation and its locus are preferably selected to achieve a smooth, linear change in intensity while also facilitating a quick transition from blackout to full brightness. The dimming wheel is used in combination with an intensity measuring feed-back sensor to control the dimming. Nevertheless, as there is a gradual change in the dimming capacity of the variable dimming wheel over the cross section of the impinging light, the dimming means introduces, albeit limited, unwanted inhomogeneities in the brightness distribution over the screen of the projection system, especially if the dimming means is not positioned in the aperture plane of the system and the light beam has a relatively large cross-section. These problems, indicated for optical radiation such as light, also are present for other types of electromagnetic radiation. Therefore, there is a need for a solution to obtain dimming of the electromagnetic radiation output of an E.M. radiation source, without substantially influencing the other properties of the E.M. radiation. This is especially the case for dimming of the light output of the light source in a projection system without substantially influencing the other properties of the light output of the light source, even if the dimming means is not positioned in the aperture plane.